New oxide superconductors

Abstract

Crystals of the new high temperature superconductor Bi₂Sr_3−xCa_xCu₂O_8+y have been grown and studied. Single crystal x-ray diffraction data were used to determine a pseudo tetragonal structure based on an A-centered orthorhombic cell with a = 5.399Å,b= 5.414Å,and c= 30.904Å. The structure contains the copper-oxygen sheets as in La₂CuO₄ and YBa₂Cu₃O₇, but the copper-oxygen chains present in YBa₂Cu₃O₇ do not occur in Bi₂Sr_3−xCa_xCu₂O_8+y. Electrical measurements on our single crystals show a resistivity drop at about 116 K and apparent zero resistivity at 91 K. We find superconductivity at 120 K in the Tℓ/Ca/Ba/Cu/O system. Also, we have for the first time prepared phases of the type La_2−xNa_xCuO₄. These materials are superconducting with T_c's as high as 40 K, and they have some unusual magnetic behavior starting at about 10 K.